In this study, water separation from water-in-oil emulsion was investigated in an oil treater using direct current electric field. A commercial motor oil was used as a model oil. Water-in-oil emulsions including 15 wt% water and 85 wt% oil were prepared with an ultrasonic generator at 27,000-28,000 Hz frequency. The performance of oil treater was evaluated in terms of water separation efficiency by changing the current, voltage, temperature, and residence time. The water separation was hardly affected by the electric current. The optimal operating voltage range was from 2 to 5 kV. The highest water separation efficiency of 77.2% was achieved at 75 ℃. The efficiency of water separation tends to increase as the temperature increases and the residence time decreases due to decreasing viscosity and increasing exposure frequency.